Writing instrument



FIG. 1 is a right side view of a writing instrument according to this design;

FIG. 2 is a rear view thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a plan view thereof;

FIG. 5 is a bottom view thereof; and,

FIG. 6 is a right front perspective view thereof. 

The ornamental design for a writing instrument, as shown. 